Boolean Difference only cuts my main object, doesn't delete any vertices
I've tried it with the default torus and default cylinder and got the same problem. I resolved this with several methods: moving the cylinder just a little off the center or scaling it up or down just a little. I guess the cylinder cuts exactly through the vertices of the torus and maybe Blender has some issues calculating it correctly. Also adding a Subdivision Surface modifier. Nearly everything helps except for keeping it at default.
EDIT: Well it doesn't cut exactly through the vertices becauses the objects have different resolutions. But still the edges overlap each other... I guess that's why. And just for clarification: I only have this problem in Blender 2.83.5, not in 2.92.0 or 2.93.0 Beta.